1. Field of the Invention
The present invention relates to phased array antenna systems which are arranged to scan over a wide angle of an orbital arc segment from a terrestrial ground station to access or track satellites within the arc segment and, more particularly, to phased array antenna systems which provide wide angle linear scan capability by orienting the phased array antenna system in a predetermined manner relative to the local terrestrial coordinate system and then squinting the beam towards the orbital arc segment.
2. Description of the Prior Art
With high capacity satellite communication systems as with subscription program satellite systems vendors or users, ground stations may wish to communicate with two or more satellites positioned at different locations along the Geosynchronous Equatorial Arc (GEA). At present, a separate ground station antenna would be used to communicate with each satellite of the system making ground stations more complex and costly. A single antenna that can track or simultaneously or sequentially communicate with all satellites of interest could circumvent the above problems.
Movable antennas of the type disclosed in, for example, U.S. Pat. Nos. 3,836,969 issued to D. S. Bond et al on Sept. 17, 1974 and 3,945,015 issued to M. Gueguen on Mar. 16, 1976 could be used for tracking purposes or for communicating with one or more satellites, but such type antennas are not useful when fast switching between multiple satellites is required. Multibeam reflector antennas using separate feedhorns as disclosed, for example, U.S. Pat. Nos. 3,914,768 issued to E. A. Ohm on Oct. 21, 1975 and 4,145,695 issued to M. J. Gans on Mar. 20, 1979 or using phased arrays as disclosed, for example, in U.S. Pat. Nos. 3,340,531 issued to G. P. Kefalas et al on Sept. 5, 1967 and 3,806,930 issued to J. F. Gobert on Apr. 23, 1974 have also been suggested for satellite ground stations. In some of such type antennas, oversized reflectors may be required while the scanning capability of others may be limited by excessive gain loss. With some of the specially designed and aberration correcting multireflector antennas with multiple feeds, for example, for a 0.5 degree beamwidth and 45 degrees of GEA coverage, a .+-.45 beamwidth scan capability is required. Such severe requirement introduces an antenna gain loss of 1 dB or more due to phase aberrations, as well as imposing a cumbersome antenna structure.
The problem, therefore, remaining in the prior art is to provide an antenna having wide angle scan capabilities which circumvents the gain loss experienced by prior art antennas while simplifying the antenna structure.